Method for preparing plastic lancet

ABSTRACT

A plastic article having a sharp point is made of a polymer of high impact strength and high hardness. The article may be a lancet or a needle for skin puncture, suturing or sewing. A method for preparing the article includes shear thinning a melt of a polymer and introducing the shear thinned melt into a mold which includes a sharp point.

This is a division of application Ser. No. 07/495,670, filed Mar. 19,1990 now U.S. Pat. No. 5,250,066.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a plastic article having a sharp point, andmore particularly relates to a medical article for tissue penetrationand a method for its preparation.

2. Background

Many articles require a sharp point for penetration of a substrate or amaterial. Such articles may, for example, be in the form of a solidneedle used in sewing or, in the surgical arena, for suturing. In themedical arts, articles having a sharp point, known as lancets, are usedto finger prick a patient's skin to draw a small blood sample.

Lancets have been employed for many years to make a quick puncture orpenetration of a patient's skin in order to provide a small outflow ofblood. Various tests may be employed with only a small amount of bloodso that the blood flowing from a finger prick is normally sufficient tocarry out a substantial number of tests. However, due to the sensitivenerve endings in the fingertip area, this procedure sometimes induces asignificant amount of pain in the patient, even though the skin punctureproduces minimal cutting. Moreover, as will be understood, many peopleare frightened by the appearance of a blade or skin puncturing apparatusof any kind wherein the cutting portion is visible to them prior to thepuncture. In order to minimize potential pain, as well as to reduceapprehension in a patient, it is desirable to make the thrust of thelancet through the patient's skin as accurately and rapidly andwithdrawal from the skin as quickly and quietly as possible. As aresult, a variety of designs for lancets which have structure forautomatic thrust and withdrawal have been disclosed, as exemplified byBurns in U.S. Pat. No. 4,677,979.

Hitherto, needles and lancets have primarily been made of metal, mostoften stainless steel, to impart the desired stiffness and strength.Metal needles, however, are manufactured by a multi-step process whichincludes drawing a metal rod to the proper size, cutting, sharpening,etching, cleaning, assembling, and connecting to a plastic holderportion. In construction of articles of the type utilized by the medicalprofession, it is often necessary to connect dissimilar materials. Thiscreates problems in those instances where the dissimilar materials arenot readily receptive for interconnection. For example, some plasticssuch as polytetrafluoroethylene do not bond well to metal componentsbecause of the resistance of polytetrafluoroethylene to most epoxies.

It is evident that medical grade articles intended for tissuepenetration will of necessity be expensive. For example, a lancetconsisting of a stainless steel blade attached to a plastic holder costsabout five cents, a prohibitive cost for a single-use, throwaway itemused in quantity as is favored in today's medical practice.

Because of the above drawbacks of metal needles and lancets, much efforthas been expended in attempts to fabricate these items by single stepinjection molding of low cost plastics. Applicants are aware of aplastic lancet of injection molded polyacetal. This material, however,is soft and compliant and undesirable for plastic puncture tips becauseits flexibility requires a substantially perpendicular angle between thepoint and the finger to prevent bending and unsuccessful puncture.Accordingly, the aforementioned polyacetal lancet is molded in the shapeof a tetrahedron to overcome the flexibility. This, however, causescutting by four edges rather than the two cutting edges of conventionalbeveled points and leads to increased pain for the patient.

Accordingly, there is a very real need in the medical profession for alancet capable of penetrating a patient's skin with minimum discomfortyet which is sufficiently inexpensive for economical single use. Thepresent invention is directed to fulfillment of this need.

SUMMARY OF THE INVENTION

One aspect of the invention is a plastic article terminating in a sharppoint molded from a polymer of high impact strength and hardness. Thearticle may be a lancet for puncturing the skin of a patient, a needleused in sewing or suturing or an insertion device for a catheter.Preferred articles are integral and include a body portion for graspingand a needle portion terminating in the sharp point.

The polymer of the article is thermoplastic and forms a melt ofsufficiently low viscosity to completely fill a mold which narrows downto a sharp point. Preferred polymers are liquid crystalline polymers andimpact resistant acrylics.

Another aspect of the invention is a method for preparing the article ofthe invention. The method includes forming a melt of the polymer ofsufficiently low viscosity to completely fill a mold which includes asharp point, cooling the melt and removing the article from the mold. Apreferred method includes shear thinning the polymer melt, preferablyunder pressure, to achieve the desired viscosity prior to filling themold. In the present disclosure, the term shear thinning describes thewell-known propensity of most polymer melts to undergo a reduction inviscosity when subjected to conditions of high shear.

Thus, the invention provides an article of a thermoplastic polymer, suchas a lancet or a needle, having a point sharp enough to penetrate apatient's skin without causing substantially more discomfort to thepatient than use of a conventional stainless steel needle. The articleof the invention is manufactured by injection molding so that it can beof any desired shape depending on the mold selected. Injection moldingis a simple, efficient and inexpensive process readily adapted to massproduction which greatly reduces the cost of the article relative tosimilar articles prepared from other materials by other processes.

The high impact strength, high hardness polymers useful in the inventionhave exceptional tensile strength, low brittleness and high bend modulusproperties very desirable in a needle. Further, the liquid crystalarticle has a very low-friction surface which facilitates mold releaseand greatly contributes to patient comfort when used for skinpenetration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lancet of the invention; and

FIG. 2 is a perspective view of a needle of the invention.

DETAILED DESCRIPTION

While this invention is satisfied by embodiments in many differentforms, there will herein be described in detail preferred embodiments ofthe invention, with the understanding that the present disclosure is tobe considered as exemplary of the principles of the invention and is notintended to limit the invention of the embodiments illustrated anddescribed. The scope of the invention will be measured by the appendedclaims and their equivalents.

In accordance with the present invention, plastic devices tapering to asharp point are fabricated from hard, thermoplastic polymers of highimpact strength by injection molding techniques. It has been found thatmelts of such polymers, preferably after shear thinning, will completelyfill a mold, including the tip, whereby a unitary plastic device havinga sharp tip may be made in a single molding operation.

Without wishing to be limited thereby, representative devices of theinvention are finger-prick lancets, solid needles, introducers forcatheters and hypodermic needles. A preferred device is a lancetcomprising a body portion for grasping and a needle portion having a tipsharp enough to penetrate a patient's skin without substantialdiscomfort or attendant breaking or bending.

Adverting now to the drawings for a more detailed description of thedevice of the invention, the figures show a lancet 10 and a needleassembly 20. Lancet 10 has integral body portion 12 for grasping andneedle portion 14 projecting therefrom and tapering to beveled point 16.Needle assembly 20 has body portion 22 and needle portion 24 tapering topoint 26 and may optionally include an eye 28.

It is not intended that the device of the invention be limited to theshapes illustrated in the drawings. Thus, body portion 12 of lancet 10,while shown to be rectangular, may be of any shape convenient forgrasping by a technician or insertion into an automatic lancet device.Likewise, the body portion 22 of needle 20, while shown to becylindrical, may be of any other convenient shape. The needle point,while illustrated to have the preferred beveled shape, may be of anysuitable design. It is also evident that the article may have otherintegrally molded parts which may be useful for the particular useintended for the article.

Polymers which may be used for molding the article of the invention havehigh impact strength and durometer hardness. Impact strength is an artterm substantially synonymous with nonbrittleness meaning the ability ofthe material to accept a sudden blow or shock without fracture. Impactstrength may be determined by Izod testing using ASTM procedure D256.Durometer hardness may be determined by ASTM procedure D2240. Inaccordance with the invention, it has been found that thermoplasticpolymers or mixtures thereof having a durometer hardness of at least 70Shore D and an Izod impact strength of at least 1.0 ft-lb/in aresuitable for the article of the invention.

Without being limited thereby, the following list of commerciallyavailable polymers may be used: 1) liquid crystalline polymers,preferably polyesters, such as Vectra™ (Hoechst-Celanese, Chatham,N.J.), Xydar™ (Amoco Performance Products, Ridgefield, Conn.), and LCP™(RTP Co., Winona, Minn.); 2) modified acrylic, such as Plexiglas™ MI-7(Rohm and Haas Co., Philadelphia, Pa.); 3) impact resistant polystyrenesuch as Rovel™, (Dow Chemical Co., Midland, Minn.), and H5M polymer(Amoco Chemical Co., Chicago, Ill.); 4) impact resistant polycarbonatesuch as Xenoy™, (General Electric Co., Pittsfield, Mass.); and 5)polyurethane having a hard segment content of 90% or greater such asIsoplast™, (Dow Chemical, Midland, Mich.).

It has been found, as described later, that liquid crystalline polymericarticles of the invention have certain processing advantages over otherpolymers. However, in terms of overall performance and unit cost, themost preferred material at the present time is modified acrylic.

The molecules in melts of most polymers exist in a completely randomstate. It is known that when polymeric melts are passed through anorifice, the elongated molecules of the polymer orient themselves intoan arrangement in which they are aligned substantially completely in thedirection of melt flow and thereby undergo a reduction in viscosity, aprocess conventionally known as shear-thinning. In accordance with theinvention, it has been found that shear thinned polymeric melts willflow into and completely fill a mold which includes a sharp tip.

When the polymers of high impact strength and hardness as defined aboveare melted, shear-thinned, directed into a pointed mold and cooled, thepolymer retains its mechanical properties. Thus, in accordance with theinvention, the article of the invention, by combining hardness andimpact strength, exhibits low flex or bend modulus and low elongation sothat the point will puncture the skin rather than bend, and high tensilestrength and impact strength to prevent the point from breaking offduring or after penetration.

The tensile strength of the molded polymer may be about 8,000 to 70,000psi, preferably about 25,000 to 35,000 psi. Elongation to break may beabout 1 to 50, preferably about 2 to 20%. The flex modulus may be about500,000 to 20,000,000, preferably about 800,000 to 7,000,000 psi. Forcomparison purposes, these properties for stainless steel are about100,000 psi, 50% and 28,000,000 psi respectively. For ordinarypolystyrene, a product totally unsuitable for the article of theinvention because of its brittleness, the values are about 6,000 psi, 2%and 480,000 psi respectively.

For ease in removal of the molded article from the mold, a conventionalmold releasing agent, such as Sprayon (Sherwin-Williams) or FluorocarbonRelease Agent Dry Lubrican (Miller-Stephenson) may preferably be used.Articles molded from shear thinned liquid crystalline polymers have avery low coefficient of friction, generally in the range of 0.1 to 0.25.This property enables the liquid crystalline article of the invention torelease from the mold without any mold releasing agents.

Another aspect of the invention is a method to prepare plastic articleshaving a sharp point. In its broadest scope, the method includes meltinga thermoplastic polymer and introducing the melt into an injection moldof the desired shape in a conventional molding operation. Molding may beperformed at any temperature between the melting and decompositiontemperatures of the polymer at which the viscosity of the melt issufficiently low to enable the melt to completely fill the mold. Toattain the highest molecular orientation and mechanical properties, aliquid crystalline polymer should be processed within its anisotropicmelt range.

In a preferred method of the invention, the polymer melt may be forcedunder pressure through an orifice and thence directly into the mold.Passage of the melt through the orifice effects shear thinning andreduces the viscosity of the melt to facilitate complete filling of themold.

Alternatively, the melt may be shear thinned, as known in the art, byblending the melt with a shear thinning additive such aspoly(α-methylstyrene) (PAMS). The PAMS may be blended into the melt inabout 1 to 10, preferably about 2 to 4, weight percent.

The degree of shear thinning is a function of the nature of the polymer,the temperature, the pressure applied, if any, and thus the rate ofpassage through the orifice, and the size of the orifice. Thesevariables are well-known in the injection molding art, and a suitablecombination of thinning and molding conditions may easily be determinedby those skilled in the art. Thus, without wishing to be limitedthereby, preferred molding parameter ranges are a pressure of 500 to1,500 psi, a shear rate of 10 to 150 sec⁻¹, and an orifice size of 0.5to 2.0 mm. After shear thinning, the polymer melt flows into andcompletely fills the mold, including the tip.

After release from the mold, the molded point of the article isgenerally sharp enough to puncture a patient's skin with minimumdiscomfort. If desired, however, the molded point may be machined by anyconventional procedure such as grinding, sanding, or may be thermoformedin a heated tipping die to further sharpen the point. A conventionallubricant may be then applied to its surface. Liquid crystallinearticles of the invention often have sufficient inherent lubricity toallow skin puncture without any lubricant.

The finished article may then be sterilized by any convenient procedure,such as chemical, heat or irradiation.

EXAMPLE I General Procedure for Molding Plastic Pointed Articles

The polymer was melted and forced at a suitable temperature and pressurethrough a die having a circular orifice of about 1.6 mm diameter. Thedie was mounted on a conventional injection molding apparatus so thatthe polymer, after passing through the orifice, flowed directly into aneedle mold having a beveled point. After filling, the mold was cooledand the article was removed. It was found that the needle had a pointwhich was comparable in penetration force to a Microfine™ stainlesssteel lancet (Becton, Dickinson and Company) when tested in accordancewith the procedure of Example III.

EXAMPLE II

Plastic lancets having sharp beveled points were prepared as describedin Example I from various polymers. Penetration forces of these lancetsare given in Table I.

                  TABLE I                                                         ______________________________________                                                     TIP PENETRATION                                                                              PENETRATION                                                    FORCE.sup.(a)  FORCE.sup.(b)                                     POLYMER      (grams)        (grams)                                           ______________________________________                                        1. Vectra ™                                                                             19             20                                                2. Polyacetal                                                                               7             15                                                3. Isoplast ™ 301                                                                        7             17                                                4. Plexiglas ™ MI-7                                                                     10             17                                                5. Microfine ™.sup.(c)                                                                  --             11                                                ______________________________________                                         .sup.(a) The initial force the cutting edge (beveled point) exerts when       cutting through the test material.                                            .sup.(b) The force required for 50% of the lancet to penetrate into the       test material.                                                                .sup.(c) Stainless steel control lancet (Becton, Dickinson and Company). 

EXAMPLE III Measurement of Penetration Force

The plastic lancet was gripped in the upper jaw of the Instron UniversalTesting Machine Model 1122 and lowered into the surface of an arbitrarypenetration medium (a disposable polyethylene 1 mm thick glove) at acrosshead speed of 10 mm/min. The force required for initial tippenetration was recorded. The lancet was then advanced into thepenetration medium until one half of the tip had penetrated and therequired force measured again.

EXAMPLE IV Measurement of Catastrophic Failure

Non brittleness or break resistance of the plastic lancet article wastested in a yes-no format. Using the setup and instrumentation asdescribed in Example III, the point of the plastic lancet was loweredinto the surface of a flat steel plate at a crosshead speed of 20mm/min. If the tip fragmented, shattered or otherwise broke apart, a"yes" to catastrophic failure was assigned. If the tip bent but stayedin one piece, a "no" to catastrophic failure was assigned. The resultsof this experiment are given in Table II.

                  TABLE II                                                        ______________________________________                                        Polymer          Catastrophic Failure                                         ______________________________________                                        1) Vectra ®  No                                                           2) Plexiglas ™ MI-7                                                                         No                                                           3) Polystyrene*  Yes                                                          4) Microfine ™                                                                              No                                                           ______________________________________                                         *PS 202 (Huntsman Chemical Co., Chesapeake, Virginia)                    

Thus, the invention provides plastic articles having a pointsufficiently strong, nonbrittle and sharp for comfortable penetration ofa patient's skin with no danger of bending or breaking during use. Thearticles are made by a simple and economical injection molding process.

What is claimed is:
 1. A method for preparing a plastic lancet having a point comprising:a) heating a thermoplastic polymer selected from the group consisting of a liquid crystalline polymer, impact resistant acrylic, impact resistant polystyrene and polyurethane having a hard segment of about 90% or greater to give a melt; b) passing said melt through an orifice whereby said melt undergoes shear-thinning, and decreases in viscosity; c) directing said melt of reduced viscosity into a mold which includes a sharp point whereby said melt completely fills said mold; d) cooling said melt in said mold whereby said melt solidifies to an article having the shape of said mold including a sharp point; and e) removing said article from said mold.
 2. The method of claim 1 wherein said melt is passed through said orifice under pressure.
 3. The method of claim 1 further comprising machining said point to further sharpen said point.
 4. The method of claim 1 further comprising thermoforming said point in a heated tipping die to further sharpen said point.
 5. A method for preparing a plastic lancet having a point comprising:a) directing a melt of a shear-thinned thermoplastic polymer having a durometer hardness of at least 70 Shore D and an impact strength of at least 1.0 ft-lbs/in into a mold which includes a point whereby said melt has a viscosity low enough to completely fill said mold; b) cooling said melt in said mold whereby said melt solidifies to an article having the shape of said mold; and c) removing said article from said mold.
 6. A method for preparing a plastic lancet comprising:a) heating an impact resistant, thermoplastic acrylic to give a melt; b) forcing said melt under pressure through an orifice whereby said melt undergoes shear-thinning and decreases in viscosity; c) directing said melt of reduced viscosity into a mold which includes a point whereby said melt completely fills said mold; d) cooling said melt in said mold whereby said melt solidifies to a lancet having the shape of said mold; and e) removing said lancet from said mold. 